Technical Abstract:
Comparative studies involving belowground carbon and nitrogen pools beneath bunch and rhizomatous grasses are needed to increase our understanding of mechanisms responsible for conferring competitive advantages to these contrasting growth forms at various locations along precipitation and soil fertility gradients. We utilized Schizachyrium scoparium (bunchgrass) and Andropogon gerardii (rhizomatous) plants in a tallgrass community, and Bouteloua gracilis (bunchgrass) and Pascopyrum smithii (rhizomatous) plants in a shortgrass community for comparisons of nutrient pools in long-term (> 25 yrs) grazed and ungrazed sites. Our hypotheses were 1) the magnitude of nutrient pools in rhizomes is comparable to that in soils beneath bunchgrasses, and 2) grazing induces a comparable reduction in nutrient pools within rhizomes and in soils beneath bunchgrasses. We found that bunchgrasses accumulated substantially greater pools in soils directly beneath plants than did rhizomatous plants in rhizomes. Nutrient pools in soils beneath the bunchgrass were greater than the combined (soil + rhizome) pools beneath the rhizomatous grass in the tallgrass community, but this relationship was reversed in the shortgrass community. Larger nutrient pools beneath the rhizomatous than the bunchgrass in the shortgrass community may have partially resulted from the occurrence of P. smithii on finer textured soils compared to B. gracilis, rather than from the direct consequences of greater nutrient accumulation. Nutrient pools for species of either growth form were not consistently modified by long-term grazing suggesting the occurrence of species and/or system specific responses.